The present invention relates generally to bearings for rotatable shafts, and more particularly to a micro adaptive flow control foil air bearing comprising an improved multi chamber air spring that exhibits a nonlinear spring rate for controlling shaft displacement that occurs in small, high speed rotors utilized in gas turbine aircraft engines.
Gas turbine engines intended for use in highly maneuverable aircraft require shaft bearings that are of minimum size and weight yet capable of supporting the weight of the rotor shaft and turbine assembly during extreme maneuvers without deflecting to the point that the turbine blades rub against the turbine shroud. Oil lubricated rolling element bearings which accomplish this task are well known. However, certain applications have size and weight limitations that militate against the use of such oil lubricated bearings.
One solution to the size and weight problem is to use a foil air bearing in the hot section of a gas turbine engine to support the turbine end of the turbine rotor shaft. Field experience indicates that, at sea level conditions with normal loads, such known air bearings meet operating requirements. However, at high altitude and under extreme loads induced by multiple "G" maneuvers, such known air bearings permit excessive turbine rotor shaft deflection that results in the turbine rubbing its shroud, causing a loss in performance and possible damage to the engine. Excessive rotor shaft deflection in such known air bearings is due primarily to the fact that they employ a spring mounted support structure within the running gap of the bearing that exhibits a constant spring rate over all load conditions, resulting in unsatisfactory performance under extreme load conditions.